Summary

Sleep has been viewed as a maladaptive behavior because it is incompatible with activity required to acquire food, defend against predation, and mate. Yet it appears to be nearly (1) universal among birds and mammals, leading to the assumption that sleep serves an unknown but vital physiological function. However, no function that can explain the huge variation in sleep times within and between species has yet been firmly identified, although many candidates, including reversal of oxidative stress, memory consolidation (2), extension of life span, and removal of various neurotoxins, have been proposed. On page 1654 of this issue, Lesku et al. (3) show that in one species of bird, those that sleep the least gain an advantage—they produce the most offspring.